439 
Copperas Creek dam and Havana, The Mytilus taken by Petersen, as 
shown by the photographed heaps as they fell out of the bottom sampler, 
were lying upon each other on the sea bottom. 
MarinE Borrom FAauNA VALUATIONS (PETERSEN, 1911—1918) 
AVERAGES, POUNDS PER ACRE*, LARGE AREAS 
Net weightj— 
Pounds per: acre fter deductions 
Depth Acres rough weight a a3-aud 75% 
Thisted Bredning over 10 m. | 16,055 3,298 800—2,200 
Nissum Bredning over 10 m. 27,170 2,418 600—1,600 
PETERSEN’S FIGURES FOR RESTRICTED COMMUNITIES, COMPARED WITH Best ILLINOIS- 
RIverR CHANNEL, 1915 
Depth Pounds |Ounces per | Ounces per 
per acre |square yard) square inch 
Mytilus community | 2m. | 167,556 552.9 0.42 Rough weight 
Modiola community | 28m. 92,036 303.7 0.23 Rough weight 
Average  Illinois-riv- 
er channel, Cop- | |... 3,029 10 aie Shells deducted 
peras Creek dam . 
to Havana 
The Food of certain Small Bottom-Invertebrates in the River 
Channel at Havana and the General Composition of 
the Detritus 
The results of microscopical examination of the stomach and gut 
contents of a number of the commoner Gastropoda, Sphaeriidae, insect 
larvae, and others of the small bottom animals of the channel opposite 
Havana in July, 1914, suggested that settled limnetic plankton plays a 
more important role in the food of the bottom fauna than seems to be gen- 
erally recognized. The studies made call for a subdivision of the com- 
moner small bottom-animals at that place into two main groups; the one 
depending principally upon plankton, and the other more largely upon 
old detritus, though containing species that make considerable use of 
* Pounds per acre calculated by us from Petersen’s figures in grams per 0.25 m.* 
t+ Our estimates. 
